This invention relates to a color transforming method which, under two different conditions such as the use of different media, devices or reproducing conditions, performs color correction and transformation such that an image reproduced under a given condition can be reproduced optimally under the other condition.
In hard copies such as those produced by printing or soft copies such as those produced by CRT display devices, colors are generally reproduced by performing area (dot percent) or density modulation of CMYK or RGB on a support. Many methods have been proposed with a view to achieving calorimetric agreements using such devices and media. Aside from the possible problem with precision, two methods are commonly known, one being the direct performance of mapping transformation as described in Unexamined Published Japanese Patent Application No. 131920/1991 which makes use of color transforming tables that are predetermined for hard copies produced with a specified printer and under specified printing conditions, and the other method involving the determination of the most likely solution of a masking coefficient such as to achieve an agreement between calorimetric values under two conditions as described in Unexamined Published Japanese Patent Application No. 362869/1992.
However, in general system configurations, more diverse variations are desired on the basis of calorimetric reproduction. One of them is an operation by which an image reproduced under a certain condition (e.g. device, medium or reproducing condition) is transformed in a way that best matches another condition which varies significantly in terms of color reproducing characteristics. In the case of prints which are most common hard copies, the color reproducing characteristics are largely dependent on the properties of the color material (e.g. ink) or support (e.g. paper) used and in order to effect transformation between two different color reproducing conditions such that the range of color reproduction under a different condition can be utilized most effectively while preserving or appropriately correcting color tones and gradations, much time and efforts are currently required on a trial-and-error basis. For example, if the ink or paper used under a certain color reproducing condition is changed, the range of color reproduction may occasionally vary between the two cases. If this occurs, it is necessary that the calorimetric reproduction of the initial color reproducing condition be preserved as much as possible while effecting mapping to a second color reproducing space associated with the different ink or paper. To meet this need, color compression is performed if the second color reproducing space is narrower than the initial one and color extension is performed in the opposite case. The color reproducing job currently performed in the printing industry is such that the condition where a color of interest such as gray does not change at limited sacrifice of other colors is determined by setting up a scanner and other devices on a trial-and-error basis, namely, requiring much time and efforts on the side of the operator.
With a view to supplementing this skill, various apparatus and algorithms have been proposed to date but no satisfactory quality or results have been attained. See, for example, Unexamined Published Japanese Patent Application No. 112042/1985 which proposes a method that uses an apparatus capable of simulating the finish of prints on a CRT so as to determine the final condition while making comparison under differing printing conditions. However, this method has so many degrees of freedom in adjustment that it is difficult to determine the optimal final condition; what is more, it is inherently impossible for the colors of soft copies such as those obtained from CRTs to have complete agreement with the colors of hard copies such as prints, so tremendous time and efforts are required to match the reproduction from a CRT with a print under specific conditions.
Another inherent problem is that when presumably optimal color transformation is to be performed under differing conditions in such a situation that complete color space agreement is unlikely to occur, the range of color reproduction need be compressed, extended or transformed between the two conditions; however, these operations often depend on the characteristics of the color reproducing space and there is no general optimum solution.
In color space compression and extension, methods are known and often attempted to preserve, compress or extend the chroma, hue or lightness in color matching spaces such as an Lab space; however, if the range of color reproduction changes greatly between two conditions, the mismatch in graphic patterns is substantial and requires a huge burden on calculations and, in addition, there is a tradeoff between the reproduction of gradations and the fidelity in color reproduction; hence, none of the techniques currently employed for color space compression and extension have proved to be completely satisfactory from the viewpoint of practical application (see Unexamined Published Japanese Patent Application Nos. 196676/1992 and 217167/1992). Thus, color space compression and gradation transformation for the case where the color reproducing characteristics differ significantly with the characteristics of the color material and the support are not considered at all in the prior art and in almost all cases it is difficult to produce an optimally transformed image.
Matching between different ranges of color reproduction or gradation reproduction characteristics in the process of color transformation between different conditions as between different hard copies (e.g. prints) is always a topic of discussion in the prior art; however, no satisfactory means of solution has yet been attained. Hence, it is strongly needed to develop a method that not only solves the stated difficulty of the prior art but also assures high quality as the result of simpler calculations.